September is usually SF's hottest month. But is a heat wave in the forecast this year?

A smoke-filled view of San Francisco's skyline from Dolores Park on Sept. 1, 2017

A smoke-filled view of San Francisco's skyline from Dolores Park on Sept. 1, 2017

Photo: Amy Graff

Photo: Amy Graff

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A smoke-filled view of San Francisco's skyline from Dolores Park on Sept. 1, 2017

A smoke-filled view of San Francisco's skyline from Dolores Park on Sept. 1, 2017

Photo: Amy Graff

September is usually SF's hottest month. But is a heat wave in the forecast this year?

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Anyone who was in San Francisco on Sept. 1, 2017, may recall stepping outside into sauna-like conditions.

The city by the bay soared to a sweltering 106 degrees that day, reaching its highest temperature in recorded history, with records going back to 1874.

The reading was extraordinary, but meteorologists agree if the city is going to break a heat record, it's most likely going to happen in September. The month is climatologically the hottest of the year, and as we move deeper into September this year, locals may be wondering if there's a heat wave in the forecast.

Well, they're likely going to have to wonder a bit longer as a hot spell is unlikely the next week according to Scott Rowe, a forecaster with the National Weather Service. Rowe says in downtown San Francisco "forecast highs are generally expected to be at or slightly below their early September normal values." But after that, a warming trend may be on the horizon.

"Beyond seven days both the European and American model ensembles are suggesting the chance of a warming trend around the weekend after next (September 13 to 15, or so)," Rowe wrote in an email. "Confidence is low beyond seven days, and of course any specifics/details are not clear."

The chance of the mercury going above 100 degrees is actually quite slim. Just 4% of years have at least one day in September with a high that's greater than or equal to 100 degrees, going back to 1874. Hitting the low-90s is much more likely with 41% of the years recording at least one day with a high between 90 and 95 degrees.

The 30-year average high — what's known as the mean maximum temperature — for September at SF's downtown weather gauge is 70.2 degrees. By comparison, the mean maximum temperature is 68.1 for August, 66.5 for July, and 66.4 in June.

June through August are typically the hottest months for inland locations around the Bay Area, but on the coast the story changes. It's actually those sweltering inland valleys — places like Livermore — that keep the city cool.

"When the interior of the state warms up ... that's what creates our sea breeze," says Jan Null, a forecaster with Golden Gate National Weather Services. "We have the cold air over the ocean and that creates the high pressure. And the heat in the interior valleys creates low pressure, a thermal low. We always blow from high pressure to low pressure. The wind blowing over the ocean inland ... that creates the fog."

By September, the ocean temperatures trend warmer and offshore winds that blow hot air from inland areas toward the coast minimize fog. The city's natural air conditioning is turned off, allowing for occasional heat waves that stretch from the coast to inland valleys.

"We don't have as much of a pressure difference between the interiors of the state and the coastal areas," explains Null. "The sun is shifting south and not warming the land as much. We start seeing high pressure over the Great Basin, and that's what create the Diablo Winds, the northerly winds that blow the hot air inland toward the coast."